 LAPACK 3.11.0 LAPACK: Linear Algebra PACKage
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## ◆ sbdt01()

 subroutine sbdt01 ( integer M, integer N, integer KD, real, dimension( lda, * ) A, integer LDA, real, dimension( ldq, * ) Q, integer LDQ, real, dimension( * ) D, real, dimension( * ) E, real, dimension( ldpt, * ) PT, integer LDPT, real, dimension( * ) WORK, real RESID )

SBDT01

Purpose:
``` SBDT01 reconstructs a general matrix A from its bidiagonal form
A = Q * B * P**T
where Q (m by min(m,n)) and P**T (min(m,n) by n) are orthogonal
matrices and B is bidiagonal.

The test ratio to test the reduction is
RESID = norm(A - Q * B * P**T) / ( n * norm(A) * EPS )
where EPS is the machine precision.```
Parameters
 [in] M ``` M is INTEGER The number of rows of the matrices A and Q.``` [in] N ``` N is INTEGER The number of columns of the matrices A and P**T.``` [in] KD ``` KD is INTEGER If KD = 0, B is diagonal and the array E is not referenced. If KD = 1, the reduction was performed by xGEBRD; B is upper bidiagonal if M >= N, and lower bidiagonal if M < N. If KD = -1, the reduction was performed by xGBBRD; B is always upper bidiagonal.``` [in] A ``` A is REAL array, dimension (LDA,N) The m by n matrix A.``` [in] LDA ``` LDA is INTEGER The leading dimension of the array A. LDA >= max(1,M).``` [in] Q ``` Q is REAL array, dimension (LDQ,N) The m by min(m,n) orthogonal matrix Q in the reduction A = Q * B * P**T.``` [in] LDQ ``` LDQ is INTEGER The leading dimension of the array Q. LDQ >= max(1,M).``` [in] D ``` D is REAL array, dimension (min(M,N)) The diagonal elements of the bidiagonal matrix B.``` [in] E ``` E is REAL array, dimension (min(M,N)-1) The superdiagonal elements of the bidiagonal matrix B if m >= n, or the subdiagonal elements of B if m < n.``` [in] PT ``` PT is REAL array, dimension (LDPT,N) The min(m,n) by n orthogonal matrix P**T in the reduction A = Q * B * P**T.``` [in] LDPT ``` LDPT is INTEGER The leading dimension of the array PT. LDPT >= max(1,min(M,N)).``` [out] WORK ` WORK is REAL array, dimension (M+N)` [out] RESID ``` RESID is REAL The test ratio: norm(A - Q * B * P**T) / ( n * norm(A) * EPS )```

Definition at line 139 of file sbdt01.f.

141*
142* -- LAPACK test routine --
143* -- LAPACK is a software package provided by Univ. of Tennessee, --
144* -- Univ. of California Berkeley, Univ. of Colorado Denver and NAG Ltd..--
145*
146* .. Scalar Arguments ..
147 INTEGER KD, LDA, LDPT, LDQ, M, N
148 REAL RESID
149* ..
150* .. Array Arguments ..
151 REAL A( LDA, * ), D( * ), E( * ), PT( LDPT, * ),
152 \$ Q( LDQ, * ), WORK( * )
153* ..
154*
155* =====================================================================
156*
157* .. Parameters ..
158 REAL ZERO, ONE
159 parameter( zero = 0.0e+0, one = 1.0e+0 )
160* ..
161* .. Local Scalars ..
162 INTEGER I, J
163 REAL ANORM, EPS
164* ..
165* .. External Functions ..
166 REAL SASUM, SLAMCH, SLANGE
167 EXTERNAL sasum, slamch, slange
168* ..
169* .. External Subroutines ..
170 EXTERNAL scopy, sgemv
171* ..
172* .. Intrinsic Functions ..
173 INTRINSIC max, min, real
174* ..
175* .. Executable Statements ..
176*
177* Quick return if possible
178*
179 IF( m.LE.0 .OR. n.LE.0 ) THEN
180 resid = zero
181 RETURN
182 END IF
183*
184* Compute A - Q * B * P**T one column at a time.
185*
186 resid = zero
187 IF( kd.NE.0 ) THEN
188*
189* B is bidiagonal.
190*
191 IF( kd.NE.0 .AND. m.GE.n ) THEN
192*
193* B is upper bidiagonal and M >= N.
194*
195 DO 20 j = 1, n
196 CALL scopy( m, a( 1, j ), 1, work, 1 )
197 DO 10 i = 1, n - 1
198 work( m+i ) = d( i )*pt( i, j ) + e( i )*pt( i+1, j )
199 10 CONTINUE
200 work( m+n ) = d( n )*pt( n, j )
201 CALL sgemv( 'No transpose', m, n, -one, q, ldq,
202 \$ work( m+1 ), 1, one, work, 1 )
203 resid = max( resid, sasum( m, work, 1 ) )
204 20 CONTINUE
205 ELSE IF( kd.LT.0 ) THEN
206*
207* B is upper bidiagonal and M < N.
208*
209 DO 40 j = 1, n
210 CALL scopy( m, a( 1, j ), 1, work, 1 )
211 DO 30 i = 1, m - 1
212 work( m+i ) = d( i )*pt( i, j ) + e( i )*pt( i+1, j )
213 30 CONTINUE
214 work( m+m ) = d( m )*pt( m, j )
215 CALL sgemv( 'No transpose', m, m, -one, q, ldq,
216 \$ work( m+1 ), 1, one, work, 1 )
217 resid = max( resid, sasum( m, work, 1 ) )
218 40 CONTINUE
219 ELSE
220*
221* B is lower bidiagonal.
222*
223 DO 60 j = 1, n
224 CALL scopy( m, a( 1, j ), 1, work, 1 )
225 work( m+1 ) = d( 1 )*pt( 1, j )
226 DO 50 i = 2, m
227 work( m+i ) = e( i-1 )*pt( i-1, j ) +
228 \$ d( i )*pt( i, j )
229 50 CONTINUE
230 CALL sgemv( 'No transpose', m, m, -one, q, ldq,
231 \$ work( m+1 ), 1, one, work, 1 )
232 resid = max( resid, sasum( m, work, 1 ) )
233 60 CONTINUE
234 END IF
235 ELSE
236*
237* B is diagonal.
238*
239 IF( m.GE.n ) THEN
240 DO 80 j = 1, n
241 CALL scopy( m, a( 1, j ), 1, work, 1 )
242 DO 70 i = 1, n
243 work( m+i ) = d( i )*pt( i, j )
244 70 CONTINUE
245 CALL sgemv( 'No transpose', m, n, -one, q, ldq,
246 \$ work( m+1 ), 1, one, work, 1 )
247 resid = max( resid, sasum( m, work, 1 ) )
248 80 CONTINUE
249 ELSE
250 DO 100 j = 1, n
251 CALL scopy( m, a( 1, j ), 1, work, 1 )
252 DO 90 i = 1, m
253 work( m+i ) = d( i )*pt( i, j )
254 90 CONTINUE
255 CALL sgemv( 'No transpose', m, m, -one, q, ldq,
256 \$ work( m+1 ), 1, one, work, 1 )
257 resid = max( resid, sasum( m, work, 1 ) )
258 100 CONTINUE
259 END IF
260 END IF
261*
262* Compute norm(A - Q * B * P**T) / ( n * norm(A) * EPS )
263*
264 anorm = slange( '1', m, n, a, lda, work )
265 eps = slamch( 'Precision' )
266*
267 IF( anorm.LE.zero ) THEN
268 IF( resid.NE.zero )
269 \$ resid = one / eps
270 ELSE
271 IF( anorm.GE.resid ) THEN
272 resid = ( resid / anorm ) / ( real( n )*eps )
273 ELSE
274 IF( anorm.LT.one ) THEN
275 resid = ( min( resid, real( n )*anorm ) / anorm ) /
276 \$ ( real( n )*eps )
277 ELSE
278 resid = min( resid / anorm, real( n ) ) /
279 \$ ( real( n )*eps )
280 END IF
281 END IF
282 END IF
283*
284 RETURN
285*
286* End of SBDT01
287*
real function slange(NORM, M, N, A, LDA, WORK)
SLANGE returns the value of the 1-norm, Frobenius norm, infinity-norm, or the largest absolute value ...
Definition: slange.f:114
subroutine scopy(N, SX, INCX, SY, INCY)
SCOPY
Definition: scopy.f:82
real function sasum(N, SX, INCX)
SASUM
Definition: sasum.f:72
subroutine sgemv(TRANS, M, N, ALPHA, A, LDA, X, INCX, BETA, Y, INCY)
SGEMV
Definition: sgemv.f:156
real function slamch(CMACH)
SLAMCH
Definition: slamch.f:68
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